Process for the production of acetic acid

ABSTRACT

A process for the production of acetic acid by reacting carbon monoxide with methanol and/or a reactive derivative thereof in a liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide, methyl acetate, water and acetic acid characterised in that there is also present in the reaction composition a monodentate phosphine oxide compound in an amount of up to and including 200 mol per gram atom of iridium.

The present invention relates to a process for the production of aceticacid and in particular, to a process for the production of acetic acidby carbonylation in the presence of an iridium catalyst and methyliodide co-catalyst.

Preparation of carboxylic acids by iridium-catalysed carbonylationprocesses is known and is described, for example in EP-A-0786447,EP-A0643034 and EP-A-0752406.

EP-A-0786447 describes a process for reacting carbon monoxide with acarbonylatable reactant and/or an ester derivative thereof in a liquidreaction composition comprising an iridium carbonylation catalyst, ahydrocarbyl halide, water and carbonylation reaction product,characterised in that the liquid reaction composition comprises water ata concentration of 2 to 8% by weight, hydrocarbyl halide at aconcentration in the range 1 to 20% by weight and ester derivative ofthe carbonylatable reactant at a concentration in the range 1.0 to 60%by weight.

EP-A-0643034 describes a process for the production of acetic acid bycarbonylation of methanol or a reactive derivative thereof which processcomprises contacting methanol or a reactive derivative thereof withcarbon monoxide in a liquid reaction composition in a carbonylationreactor characterised in that the liquid composition comprises (a)acetic acid, (b) an iridium catalyst, (c) methyl iodide, (d) at least afinite quantity of water, (e) methyl acetate and (f) as promoter, atleast one of ruthenium and osmium.

EP-A-0752406 describes a process for the production of acetic acidcomprising (1) continuously feeding methanol and/or a reactivederivative thereof and carbon monoxide to a carbonylation reactor whichcontains a liquid reaction composition comprising an iridiumcarbonylation catalyst, methyl iodide co-catalyst, a finiteconcentration of water, acetic acid, methyl acetate and at least onepromoter; (2) contacting the methanol and/or reactive derivative thereofwith the carbon monoxide in the liquid reaction composition to produceacetic acid; and (3) recovering acetic acid from the liquid reactioncomposition characterised in that there is continuously maintained inthe liquid reaction composition throughout the course of the reaction(a) water at a concentration of no greater than 6.5% by weight, (b)methyl acetate at a concentration in the range 1 to 35% by weight and(c) methyl iodide at a concentration in the range 4 to 20% by weight.

The use of polydentate chelating phosphorus or arsenic ligands incarbonylation processes is known, for example from U.S. Pat. No.4,102,920 and U.S. Pat. No. 4,102,921 which describe respectively, theiruse in rhodium and iridium catalysed carbonylation processes.

The use of phosphine oxide promoters in rhodium-catalysed carbonylationprocesses is known from U.S. Pat. No. 5,817,869 and from EP-A-0114703.

Thus, U.S. Pat. No. 5,817,869 relates to process for the production ofacetic acid without the use of an alkali metal halide comprisingcontacting methanol or methyl acetate with carbon monoxide in thepresence of a carbonylation system containing about 200 to about 1200ppm of rhodium-containing component and a liquid reaction mediumcomprising about 20 to about 80 weight % acetic acid; from about 0.6 toabout 36 weight % methyl iodide; from about 0.5 to about 10 weight %methyl acetate, said contacting being in the presence of at least onepentavalent Group VA oxide of the formula R₃M═O, which is present in aconcentration of Group VA oxide to rhodium of greater than about 60:1,and water being added in an amount of from about 4 to about 12 weight %.

EP-A-0114703 relates to a process for the preparation of carboxylicacids and/or esters by reaction of an alcohol with carbon monoxide inthe presence of a rhodium compound, an iodide and/or bromide source anda phosphorus, arsenic or antimony-containing compound as promoter,characterised in that the reaction is carried out in the presence of acompound of the formula

wherein X represents phosphorus, arsenic or antimony and Y oxygen,sulphur or selenium and either a and b, independent of one another, are0 or 1, R¹ represents hydrogen or an unsubstituted or substitutedhydrocarbon group and R² and R³ each represent an unsubstituted orsubstituted hydrocarbon group, or a and b are 0 and R² and R³ togetherwith X form a heterocyclic group and R¹ represents hydrogen or anunsubstituted or substituted hydrocarbon group, or in the presence of acomplex of a compound of formula I with a hydrocarbon iodide or bromide,an acyl iodide or bromide or hydrogen iodide or bromide. Examples ofcompounds of formula I wherein a and b are 0, include triphenylphosphineoxide

According to EP-A-0114703, the quantity of compound of formula I used aspromoter in the process may vary within wide limits, for instancebetween 0.1 and 300 mol per gram atom rhodium. Preference is said to begiven to use of 1-200, in particular 10-100 mol per gram atom rhodium.The promoters of EP-A-0114703 are directed towards improving theactivity of the rhodium carbonylation catalyst system.

The technical problem to be solved is to provide an improvedcarbonylation process for the production of acetic acid. It has now beensurprisingly found that by using a specified quantity of a monodentatephosphine oxide compound in an iridium-catalysed carbonylation processfor the production of acetic acid, the quantities of by-productpropionic acid, its precursors and derivatives produced are reduced andhence selectivity to the desired acetic acid is increased.

Thus, according to the present invention, there is provided a processfor the production of acetic acid by reacting carbon monoxide withmethanol and/or a reactive derivative thereof in a liquid reactioncomposition comprising an iridium carbonylation catalyst, methyl iodide,methyl acetate, water and acetic acid characterised in that there isalso present in the reaction composition a monodentate phosphine oxidecompound in an amount of up to and including 200 mol per gram atom ofiridium.

The process of the present invention solves the technical problemdefined above, by the use of a monodentate phosphine oxide compound inan amount of up to and including 200 mol per gram atom of iridium toreduce the amount of by-product propionic acid, its precursors such asethyl iodide and ethyl acetate and its derivatives such as methylpropionate and ethyl propionate produced and increase the selectivity ofthe process to the desired acetic acid.

The monodentate phosphine oxide compound may be represented by theformula

wherein R¹, R², R³ are independently an unsubstituted or substituted C₁to C₁₀ alkyl group or an unsubstituted or substituted C₅ to C₁₅ arylgroup.

The C₁ to C₁₀ alkyl group may be straight or branched. Examples ofsuitable C₁-C₁₀ alkyl groups include methyl, ethyl, n-butyl and n-octyl.The C₁-C₁₀ alkyl group may be substituted by one or more substituentse.g. 1-4 substituents. Suitable substituents include C₁ to C₁₀ alkylgroups and C₅ to C₁₅ aryl groups.

The C₅-C₁₅ aryl group may be e.g. phenyl, alpha-naphthyl andbeta-naphthyl, preferably phenyl. The C₅-C₁₅ group may be substitutedwith substituents selected from the group consisting of —NO₃, —OH, —CN,—SO₃H, —OCH₃ and —CO₂H.

A preferred monodentate phosphine oxide compound of formula II istriphenylphosphine oxide.

In the process of the present invention if at least one promoter ispresent in the reaction composition, higher concentrations of themonodentate phosphine oxide compound can be used, which enables thebenefits of reduced by-product formation to be achieved at acceptablereaction rates. Suitable promoters are preferably selected from thegroup consisting of ruthenium, osmium, rhenium, cadmium, mercury, zinc,gallium, indium and tungsten, and are more preferably selected from thegroup consisting of ruthenium and osmium and most preferably isruthenium. Preferably, the promoter is present in an effective amount upto the limit of its solubility in the liquid reaction composition and/orany liquid process streams recycled to the carbonylation reactor fromthe acetic acid recovery stage. The promoter is suitably present in theliquid reaction composition at a molar ratio of promoter:iridium in therange 0.5:1 to 15:1, preferably 0.5:1 to 10:1. Any reduction incarbonylation rate caused by the presence of the monodentate phosphineoxide compound may be off-set by increasing the concentration ofpromoter.

The promoter may comprise any suitable promoter metal-containingcompound which is soluble in the liquid reaction composition. Thepromoter may be added to the liquid reaction composition for thecarbonylation reaction in any suitable form which dissolves in theliquid reaction composition or is convertible to soluble form.

The monodentate phosphine oxide compound may be present in the reactioncomposition in an amount of greater than 0.5 mol per grain atom ofiridium. Preferably, the monodentate phosphine oxide compound is presentin the reaction composition in an amount of 1 to 200 mol per gram atomof iridium when a promoter is present and is present in the reactioncomposition in an amount of 1 to 100 mol per gram atom of iridium whenno promoter is present.

In the process of the present invention, the iridium carbonylationcatalyst is preferably present in the liquid reaction composition at aconcentration in the range 400 to 5000 ppm measured as iridium, morepreferably in the range 500 to 3000 ppm measured as iridium. Anyreduction in carbonylation rate caused by the presence of themonodentate phosphine oxide compound may be off-set by increasing theconcentration of iridium catalyst, if present.

The iridium catalyst in the liquid reaction composition may comprise anyiridium containing compound which is soluble in the liquid reactioncomposition. The iridium catalyst may be added to the liquid reactioncomposition for the carbonylation reaction in any suitable form whichdissolves in the liquid reaction composition or is convertible to asoluble form.

In the process of the present invention, the concentration of methyliodide co-catalyst in the liquid reaction composition is preferably inthe range 5 to 16% by weight.

In the process of the present invention, suitable reactive derivativesof methanol include methyl acetate, dimethyl ether and methyl iodide. Amixture of methanol and reactive derivatives thereof may be used asreactants in the process of the present invention. Preferably, methanoland/or methyl acetate are used as reactants. At least some of themethanol and/or reactive derivative thereof will be converted to, andhence present as, methyl acetate in the liquid reaction composition byreaction with acetic acid product or solvent. In the process of thepresent invention the concentration of methyl acetate in the liquidreaction composition is preferably in the range 1 to 30% by weight, morepreferably 5 to 25% by weight.

Water may be formed in situ in the liquid reaction composition, forexample, by the esterification reaction between methanol reactant andacetic acid product. Small amounts of water may also be produced byhydrogenation of methanol to produce methane and water. Water may beintroduced to the carbonylation reactor together with or separately fromother components of the liquid reaction composition. Water may beseparated from other components of reaction composition withdrawn fromthe reactor and may be recycled in controlled amounts to maintain therequired concentration of water in the liquid reaction composition. Thewater concentration in the liquid reaction composition is suitably inthe range 1-15 wt %, preferably in the range 1-6.5 wt %.

Preferably, the iridium- and promoter-containing compounds are free ofimpurities which provide or generate in situ ionic iodides which mayinhibit the reaction, for example, alkali or alkaline earth metal orother metal salts.

Ionic contaminants such as, for example, (a) corrosion metals,particularly nickel, iron and chromium and (b) phosphines or nitrogencontaining compounds or ligands which may quaternise in situ; should bekept to a minimum in the liquid reaction composition as these will havean adverse effect on the reaction by generating I⁻ in the liquidreaction composition which has an adverse effect on the reaction rate.Some corrosion metal contaminants such as for example molybdenum havebeen found to be less susceptible to the generation of I⁻. Corrosionmetals which have an adverse affect on the reaction rate may beminimised by using suitable corrosion resistant materials ofconstruction. Similarly, contaminants such as alkali metal iodides, forexample lithium iodide, should be kept to a minimum. Corrosion metal andother ionic impurities may be reduced by the use of a suitable ionexchange resin bed to treat the reaction composition, or preferably acatalyst recycle stream. Such a process is described in U.S. Pat. No.4,007,130. Preferably, ionic contaminants are kept below a concentrationat which they would generate 500 ppm I⁻, preferably less than 250 ppm I⁻in the liquid reaction composition.

The carbon monoxide reactant may be essentially pure or may containinert impurities such as carbon dioxide, methane, nitrogen, noble gases,water and C₁ to C₄ paraffinic hydrocarbons. The presence of hydrogen inthe carbon monoxide feed and generated in situ by the water gas shiftreaction is preferably kept low as its presence may result in theformation of hydrogenation products. Thus, the amount of hydrogen in thecarbon monoxide reactant is preferably less than 1 mol %, morepreferably less than 0.5 mol % and yet more preferably less than 0.3 mol% and/or the partial pressure of hydrogen in the carbonylation reactoris preferably less than 1×10⁵ N/m² partial pressure, more preferablyless than 5×10⁴ N/m² and yet more preferably less than 3×10⁴ N/m². Thepartial pressure of carbon monoxide in the reactor is suitably in therange 1×10⁵ N/m² to 7×10⁶ N/m², preferably 1×10⁵ N/m² to 3.5×10⁶ N/m²,more preferably 1×10⁵ N/m² to 1.5×10⁶ N/m².

The total pressure of the carbonylation reaction is suitably in therange 1×10⁶ N/m² to 2×10⁷ N/m², preferably 1.5×10⁶ N/m² to 1×10⁷ N/m²,more preferably 1.5×10⁶ N/m² to 5×10⁶ N/m².

The temperature of the carbonylation reaction is suitably in the range100 to 300° C., preferably in the range 150 to 220° C. Any reduction incarbonylation rate caused by the presence of the monodentate phosphineoxide compound may be off-set by increasing the reaction temperature.

The process of the present invention is preferably performed as acontinuous process.

The acetic acid product may be recovered from the liquid reactioncomposition by withdrawing vapour and/or liquid from the carbonylationreactor and recovering acetic acid from the withdrawn material.Preferably, acetic acid is recovered from the liquid reactioncomposition by continuously withdrawing liquid reaction composition fromthe carbonylation reactor and recovering acetic acid from the withdrawnliquid reaction composition by one or more flash and/or fractionaldistillation stages in which the acetic acid is separated from the othercomponents of the liquid reaction composition such as iridium catalyst,methyl iodide co-catalyst, promoter, methyl acetate, unreacted methanol,water and acetic acid solvent which may be recycled to the reactor tomaintain their concentrations in the liquid reaction composition. Tomaintain stability of the iridium catalyst during the acetic acidproduct recovery stage, water in process streams containing iridiumcarbonylation catalyst for recycle to the carbonylation reactor shouldbe maintained at a concentration of at least 0.5% by weight.

The process of the present invention may be performed a usingcarbonylation reaction conditions known in the art, for example asdescribed in EP-A-0786447, EP-A-0643034, EP-A-0752406 and EP-A-0749948,the contents of which are hereby incorporated by reference.

The invention will now be illustrated by way of example only and withreference to the following examples:

GENERAL REACTION METHOD

A 300 cm³ zirconium autoclave, equipped with a stirrer and a liquidinjection facility, was used for a series of batch autoclaveexperiments. The autoclave was flushed up to 5×10⁵ N/m² with nitrogenand then flushed with carbon monoxide up to 5×10⁵ N/m². An initialcharge consisting of methyl acetate, acetic acid, methyl iodide andwater, was placed into the autoclave. A catalyst solution was primedinto a liquid injection line made up of (1.35 g) of H₂IrCl₆ aqueoussolution (22.26% Ir w/w), water (5.0 g) and acetic acid (10.0 g). Theresulting combined autoclave charge was thus methyl acetate (60.02 g),acetic acid (48.84 g), methyl iodide (23.77 g) and water (17.81 g). Thecontents of the autoclave were heated to 190° C. whilst being stirred at1500 rpm. Once the temperature had reached 190° C. the H₂IrCl₆ catalystsolution was injected into the autoclave with an over-pressure of carbonmonoxide to the hot autoclave to bring the autoclave pressure to 2.8×10⁶N/m².

The autoclave pressure and temperature were maintained at a constant2.8×10⁶ N/m² and 190° C. respectively throughout the reaction, until theuptake of carbon monoxide from a ballast vessel ceased. On terminationof the reaction the autoclave was cooled and vented. After cooling andcarefully venting the autoclave the liquid components were dischargedand analysed for liquid products and by-products by known establishedgas chromatography methods.

Liquid by-products are determined by gas chromatography using a CB wax52column on a Hewlett Packard 6820 Mk2 gas chromatograph. Detectedcomponents are quantified by integration of the component peaks relativeto an external standard and expressed as parts per million (ppm) byweight.

The main liquid by-product from carbonylation of methanol to acetic acidis propionic acid. It's precursors (ethyl iodide and ethyl acetate) arealso formed. In a continuous process these precursors would be recycledto the carbonylation reactor in recycle streams, building up to a steadystate concentration at which the rate of their destruction to propionicacid balances their rate of removal. In a batch process, theseprecursors are not destroyed, but accumulate with the propionic acid inthe liquid reaction composition and these can be measured at the end ofthe experiment. A reduction in the amount of propionic acid and itsprecursors measured at the end of a batch carbonylation experiment wouldbe expected to indicate that in a continuous process, the amount ofby-product propionic acid recovered with the acetic acid product wouldalso be reduced.

In the batch reactions ‘Total’ propionic acid was defined as the sum ofpropionic acid and it's precursors ((ethyl acetate and ethyl iodide)converted to ppm propionic acid), detected in the quenched liquidproducts of the batch reaction and expressed in ppm.

Total propionic acid=ppm propionic acid+(ppm ethyliodide×(74.08/155.97))+(ppm ethyl acetate×(74.08/88.11))

This represents the cumulative formation during the batch experiment ofpropionic acid and its precursors, ethyl iodide and ethyl acetate.Ethanol and acetaldehyde are produced in very small amounts such thatthey can be ignored.

The rate of gas uptake at a certain point in a reaction run was used tocalculate the carbonylation rate, as number of moles of reactantconsumed per liter of cold degassed reactor composition per hour(mol/l/hr), at a particular reactor composition (total reactorcomposition based on a cold degassed volume).

The methyl acetate concentration was calculated during the course of thereaction from the starting composition, assuming that one mole of methylacetate was consumed for every mole of carbon monoxide that wasconsumed. No allowance was made for organic components in the autoclaveheadspace.

By monitoring the rate of carbonylation reaction and calculating theconcentration of the reaction components during the experiment, it ispossible to determine the rate of carbonylation reaction which would beexpected if a carbonylation process were to be operated continuouslywhilst maintaining under steady state, a liquid reaction compositionwhich is the same as the total reaction composition calculated at anyparticular point in the batch experiment. In the batch experiments theterm ‘reaction composition’ refers to the total composition of thecomponents in the autoclave in the cold degassed state. The principaldifference between the batch experiments and continuous operation isthat in the batch experiments, no allowance was made in calculating thecomponent concentrations, for partitioning of the reaction componentsbetween the liquid and gaseous phases, Owing to this partitioning, theconcentration of the reaction components present in the liquid phase ina batch reaction under reaction conditions was similar, but notidentical, to the total reaction composition. In particular, the morevolatile components in the reaction composition, such as methyl iodideand methyl acetate, were slightly less concentrated in the liquidreaction composition than in the total reaction composition, whereas thewater concentration was comparable between the two. Therefore, the ratecalculated in a batch experiment at a certain total reaction compositionwill be similar to that in a continuous process operating with a liquidcomposition which is the same as the batch total reaction composition.In addition, trends observed in batch experiments by varying the processvariables, such as water concentration, were comparable with the trendsobserved in continuous experiments.

EXPERIMENT A

A baseline experiment was performed using 1.35 g of H₂IrCl₆ solution anda combined autoclave charged of methyl acetate (60.02 g), acetic acid(48.84 g), water (17.81 g) and methyl iodide (23.77 g).

The rate of reaction, based on carbon monoxide uptake was measured to be10.04 mol/l/hr at a calculated reaction composition of 12.5% methylacetate and steadily declined until virtually all the methyl acetate wasconsumed. Conversion to acetic acid was 98.2% based on methyl acetateconsumed. Analysis of propionic acid precursors gave a total propionicacid make of 604 ppm. Gaseous by-products in the cold-vented off-gaswere H₂, 1.94% v/v; CO₂, 2.24% v/v; CH₄, 3.67% v/v. Analysis of thefinal reaction composition by Gas Chromatography gave methyl iodide13.8%; methyl acetate 0.59%; acetic acid 90.34%.

This is not an example according to the present invention because nomonodentate phosphine oxide was present.

EXAMPLE 1

Experiment A was repeated except that the total combined autoclavecharge consisted of methyl acetate (59.99 g), acetic acid (47.80 g),water (17.81 g) and methyl iodide (23.77 g). Triphenylphosphine oxide(43.40 g) was charged to the autoclave and 1.351 g of H₂IrCl₆ solutionwas used in the catalyst charge. The rate of reaction at a calculatedreaction composition of 12.5% methyl acetate was measured as approx. 1.3mol/l/hr. Conversion to acetic acid was 94.3% based on methyl acetateconsumed. Analysis of the propionic acid precursors gave a totalpropionic acid make of 43 ppm. Gaseous by-products in the cold-ventedoff-gas were not measured due to a leak. Analysis of the final reactioncomposition by Gas Chromatography gave methyl iodide 1.65%; methylacetate 41.1%; acetic acid 45.4%.

This is an example of the use of triphenylphosphine oxide at 200 mols.per gram atom of iridium catalyst and shows the reduction in liquid andgaseous by-products.

EXAMPLE 2

Experiment A was repeated except that the total combined autoclavecharge consisted of methyl acetate (60.03 g), acetic acid (42.80 g),water (22.84 g) and methyl iodide (23.78 g). Triphenylphosphine oxide(10.85 g) was charged to the autoclave and 1.349 g of H₂IrCl₆ solutionwas used in the catalyst charge. The rate of reaction at a calculatedreaction composition of 12.5% methyl acetate was measured as 3.57mol/l/hr. Conversion to acetic acid was 82.7% based on methyl acetateconsumed. Analysis of the propionic acid precursors gave a totalpropionic acid make of 185 ppm. Gaseous by-products in the cold-ventedoff-gas were H₂, 0.37% v/v/; CO₂, 3.53% v/v; CH₄, 8.5% v/v. Analysis ofthe final reaction composition by Gas Chromatography gave methyl iodide3.8%; methyl acetate 5.75%; acetic acid 73.08%.

This is an example of the use of triphenylphosphine oxide at 50 mols pergram atom of iridium catalyst to reduce liquid and gaseous by-products.

EXAMPLE 3

Experiment A was repeated except the total combine autoclave chargeconsisted of methyl acetate (60.0 g), acetic acid (48.28 g), water(17.75 g) and methyl iodide (23.77 g). Triphenylphosphine oxide (2.16 g)was charged to the autoclave and 1.348 g of H₂IrCl₆ solution was used inthe catalyst charge. The rate of reaction at a calculated reactioncomposition of 12.5% methyl acetate was measured as 6.60 mol/l/hr.Conversion to acetic acid was 92.2% based on methyl acetate consumed.Analysis of the propionic acid precursors gave a total propionic acidmake of 278 ppm. Gaseous by-products in the cold-vented off-gas were H₂,0.94% v/v; CO₂, 2.57% v/v; CH4, 5.94% v/v. Analysis of the finalreaction composition by Gas Chromatography gave methyl iodide 1.91%;methyl acetate 2.67%; acetic acid 78.42%.

This is an example of the use of triphenylphosphine oxide at 10 mols pergram atom of iridium catalyst to reduce liquid and gaseous by-products.

EXAMPLE 4

Experiment A was repeated except the total combined autoclave chargeconsisted of methyl acetate (60.01 g), acetic acid (47.79 g), water(17.85 g) and methyl iodide (23.77 g). Triphenylphosphine oxide (0.22 g)was charged to the autoclave and 1.349 g of H₂IrCl₆ solution was used inthe catalyst charge. The rate of reaction at a calculated reactioncomposition of 12.5% methlyl acetate was measured as 10.3 mol/l/hr.Conversion to acetic acid was 98.5% based on methyl acetate consumed.Analysis of the propionic acid precursors gave a total propionic acidmake of 412 ppm. Gaseous by-products in the cold-vented off-gas were H₂,1.58% v/v; CO₂, 2.42% v/v; CH₄, 5.21% v/v. Analysis of the finalreaction composition by Gas Chromatography gave methyl iodide 11.6%;methyl acetate 0.51%; acetic acid 88.95%.

This is an example of the use of triphenylphosphine oxide at 1 mols pergram atom of iridium catalyst to reduce liquid and gaseous by-products.

EXPERIMENT B

A baseline experiment was performed repeating Experiment A except thetotal combined autoclave charge consisted of methyl acetate (60.0 g),acetic acid (44.58 g), water (18.71 g), methyl iodide (12.45 g) andRu(CO)₄I₂ (3.64 g). 1.34 g of H₂IrCl₆ solution was used in the catalystcharge. The rate of reaction at a calculated reaction composition of12.5% methyl acetate was measured as 25.94 mol/l/hr. Conversion toacetic acid was 97.1% based on methyl acetate consumed. Analysis of thepropionic acid precursors gave a total propionic acid make of 293 ppm.Gaseous by-products in the cold-vented off-gas were H₂, 2.42% v/v; CO₂,2.9% v/v; CH₄, 5.63% v/v. Analysis of the final reaction composition byGas Chromatography gave methyl iodide 0.831%; methyl acetate 1.0%;acetic acid 82.42%.

This is not an example according to the present invention because nomonodentate phosphine oxide was present.

EXAMPLE 5

Experiment B was repeated except that the total combined autoclavecharge consisted of charge consisted of methyl acetate (60.03 g), aceticacid (54.53 g), water (18.80 g), methyl iodide (12.43 g), Ru(CO)41₂(3.70 g ) and triphenylphosphine oxide (0.44 g). 1.341 g of H₂IrCl₆solution was used in the catalyst charge. The rate of reaction at acalculated reaction composition of 12.5% methyl acetate was measured as25.65 mol/l/hr. Conversion to acetic acid was 96.7% based on methylacetate consumed. Analysis of the propionic acid precursors gave a totalpropionic acid make of 283 ppm. Gaseous by-products in the cold-ventedoff-gas were H₂, 2.18% v/v; CO₂, 2.88% v/v; CH₄, 5.75% v/v/. Analysis ofthe final reaction composition by Gas Chromatography gave methyl iodide0.01%; methyl acetate 1.16%; acetic acid 79.8%.

This is an example of the use of triphenylphosphine oxide at 1 mols pergram atom of iridium catalyst in presence of 5 mols ruthenium promoterper gram atom of iridium to reduce liquid and gaseous by-products.

EXAMPLE 6

Experiment B was repeated except that the total combined autoclavecharge consisted of methyl acetate (60.02 g), acetic acid (49.61 g),water (8.72 g), methyl iodide (12.44 g), Ru(CO)₄I₂ (3.65 g) andtriphenylphosphine oxide (2.17 g). 1.346 g of H₂IrCl₆ solution was usedin the catalyst charge. The rate of reaction at a calculated reactioncomposition of 12.5% methyl acetate was measured as 20.37 mol/l/hr.Conversion to acetic acid was 94.7% based on methyl acetate consumed.Analysis of the propionic acid precursors gave a total propionic acidmake of 158 ppm. Gaseous by-products in the cold-vented off-gas were H₂,0.82% v/v; CO₂, 2.95% v/v; CH₄, 5.44% v/v. Analysis of the finalreaction composition by Gas Chromatography gave methyl iodide 0.04%;methyl acetate 1.85%; acetic acid 78.4%.

This is an example of the use of triphenylphosphine oxide at 5 mols pergram atom of iridium catalyst in presence of 5 mols ruthenium promoterper gram atom of iridium to reduce liquid and gaseous by-products.

The results are summarised in the Table below.

Note: Since propionic acid formation increases rapidly above 90%conversion of methyl acetate in iridium and iridium/ruthenium catalysedcarbonylations, more accurate comparisons from batch reactions are foundin experiments that are terminated below 90% conversion.

TABLE Reaction rate at Propionic Con- 12.5% acid ver- methyl Example/Catalyst make sion acetate Experiment system Additive (ppm) (%) mol/l/hrComments Experiment A Ir none 604 98.2% 10.04 Example 1 Irtriphenylphosphine  43 94.3% 1.3 Reduced rate but reduced oxidepropionic acid. Example 2 Ir triphenylphosphine 185 82.7 3.57 Reducedrate but reduced oxide propionic acid. Example 3 Ir triphenylphosphine278 92.2% 6.6 Reduced rate but reduced oxide propionic acid Example 4 Irtriphenylphosphine 412 98.5% 10.3 Good rate and reduced oxide propionicacid Experiment B Ir/Ru none 293 97.1% 25.94 Example 5 Ir/Rutriphenylphosphine 283 96.7% 25.65 Good rate and propionic acid oxideremains low Example 6 Ir/Ru triphenylphosphine 158 94.7% 20.37 Reducedrate but reduced oxide propionic acid

I claim:
 1. A process for the production of acetic acid which comprisesreacting carbon monoxide with a reactant selected from the groupconsisting of methanol, reactive derivatives of methanol and mixturesthereof in a liquid reaction composition comprising an iridiumcarbonylation catalyst, methyl iodide, methyl acetate, water and aceticacid wherein there is also present in the reaction composition amonodentate phosphine oxide compound in an amount up to and including200 mol per gram atom of iridium.
 2. A process according to claim 1wherein the monodentate phosphine oxide compound is present in thereaction composition in an amount of greater than 0.5 mol per gram ofiridium.
 3. A process according to claim 1 wherein the monodentatephosphine oxide compound is represented by the formula:

wherein R1, R2, R3 are independently an unsubstituted or substituted C₁to C₁₀ alkyl group or an unsubstituted or substituted C₅ to C₁₅ arylgroup.
 4. A process according to claim 1 wherein the reactioncomposition further comprises at least one promoter.
 5. A processaccording to claim 4 wherein the at least one promoter is selected fromthe group consisting of ruthenium, osmium, rhenium, cadmium, mercury,zinc, gallium, indium and tungsten.
 6. A process according to claim 2wherein the reaction composition further comprises at least onepromoter.
 7. A process according to claim 6 wherein the at least onepromoter is selected from the group consisting of ruthenium, osmium,rhenium, cadmium, mercury, zinc, gallium, indium and tungsten.
 8. Aprocess according to claim 4 wherein the monodentate phosphine oxidecompound is present in the reaction composition in an amount of 1 to 200mol per gram atom of iridium.